Multiple sclerosis (MS) is a progressive neurological disorder characterized by an autoimmune mediated attack against the myelin sheath in the CNS resulting in inflammation, demyelination, gliosis and ultimately axonal degeneration [Bruck and Stadelmann 2003]. The clinical course of MS has been divided into four major categories: RR, SP, PP and benign. Patients who have clinical relapses every few months or years with intervening periods of clinical stability define RRMS. RRMS is twice as common in females than males in the second or third decade of life [Noseworthy et al. 2000]. Although the majority of MS patients are initially diagnosed with RRMS, over time increasing numbers of these individuals convert to SPMS characterized by a gradual decline in neurological function [Trojano et al. 2003]. Approximately 15% of MS patients have PPMS characterized by an absence of clinical relapses and an unrelenting deterioration of neurological function from disease onset [McDonnell and Hawkins 2002]. PPMS is characterized by relatively late-onset (mean age ˜39 years) although it has been suggested based on clinical and MRI data that the pre-clinical phase of PPMS occurs during the same time frame as in RRMS. In PPMS as in RRMS the CNS lesions have the same age of onset, but it takes 10 years for PPMS patients to develop symptoms [McDonnell et al. 2003]. MRI with the contrast agent gadolinium is often employed during or following an initial attack to identify lesions within the CNS that are consistent with a diagnosis of MS. MRI is also used at protracted intervals in MS patients to identify new areas of demyelinated plaques within the brain and spinal cord [Calabresi 2004]. Moreover, MRI activity correlates with immune cell perturbations in early possible MS. For example, Rinaldi et al. (2006) demonstrated that distinct changes in peripheral lymphocyte subsets occur over the course of 1 year, which differentiate MRI active, and MRI inactive patients following a clinically isolated syndrome. Yet results obtained by MRI do not correlate with clinical disability and are therefore not recommended as therapeutic end-points for new MS therapeutics [Siva 2006]. In addition to RRMS, SPMS and PPMS, there is a benign form of the disease that affects approximately 15% of RRMS patients. Benign MS is arbitrarily defined in RRMS patients who after more than 10 to 15 years following initial diagnosis are still mobile and show only mild deficits (EDSS≦4). Typically, these patients show little or no progression after their initial attack. Moreover, those patients with an EDSS score of ≦2 and disease duration of more than 10 to 15 years tend to maintain a low EDSS disability score for an additional 10 years. Benign MS requires no therapeutic intervention, however, it is not possible to diagnose this form of MS until at least 5 to 10 years from MS onset [Hawkins and McDonnell 1999; Pittock et al. 2004]. Unfortunately, there are no diagnostic tests that would allow a clinician to predict whether a newly diagnosed MS patient will follow a benign or aggressive disease course.
MS is considered to be a T cell-mediated autoimmune disease of the brain and spinal cord [Traugott et al. 1983; Vizler et al. 1999]. While there appears to be a localized CNS immune response, peripheral immune cell abnormalities appear to correlate with central disease activity [Hafler and Weiner 1989] and may precede MRI activity. Apoptosis is an important mechanism in immune system regulation, responsible for elimination of autoreactive T-lymphocytes (T cells), B-lymphocytes (B cells) and monocytes from the circulation and prevention of their entry into the CNS [Mahoney and Rosen 2005; Todaro et al. 2004]. It has been hypothesized that a genetic predisposition exists in MS patients whereby a failure of autoreactive T cells and B cells as well as activated macrophages to undergo apoptosis contributes to the pathogenesis of MS [Bernard and Derosbo 1992; Pender 1998; Pender and Rist 2001]. Consistent with this hypothesis expression of members of the IAP family of anti-apoptotic proteins are elevated in mitogen (PHA) stimulated T cells derived from the CSF or blood of MS patients relative to healthy or neurological control subjects [Segal and Cross 2000; Seki et al. 1988; Semra et al. 2002; Sharief et al. 2002b; Sharief and Semra 2001; Tsukamoto et al. 1986]. The IAP family of anti-apoptotic genes encodes proteins that directly bind to and inactivate initiator and effector caspases, a group of cysteinyl proteases that mediate the initiation and execution of apoptosis [Holcik et al. 2001; Salvesen and Duckett 2002]. First discovered in baculovirus, the IAPs are well conserved in eukaryotes, ranging from yeast to humans and to date, eight human IAPs have been identified [Holcik et al. 2001; Nachmias et al. 2004]. Importantly, IAPs are the only intrinsic inhibitors of caspases. The IAPs are typified by the presence of a variable number of highly conserved domains about 70 amino acids in length, known as BIR domains, which are critical for anti-apoptotic activity. For example, while XIAP, HIAP-1 and HIAP-2 contain three BIR domains, survivin possesses only one BIR domain. Although highly similar, the individual BIR domains are not functionally equivalent. The BIR2 domain and the preceding linker region of XIAP, HIAP-1, and HIAP-2 facilitate the interaction with and suppression of caspases 3 and 7; the two most potent effector caspases [Eckelman and Salvesen 2006; Nachmias et al. 2004; Robertson et al. 2000]. Inhibition of the initiatior caspase 9 is accomplished by the BIR3 domain. While XIAP, HIAP-1 and HIAP-2 possess BIR domains 2 and 3 capable of binding caspases 7 and 9, only XIAP contains critical domain residues capable of direct caspase inhibition [Eckelman and Salvesen 2006]. In addition to BIR domains, XIAP, HIAP-1 and HIAP-2 possess a carboxy-terminal RING zinc finger motif that has E3 ubiquitin ligase activity targeting caspases for degradation by proteosomes [Holcik et al. 2001]. In addition, it has been shown recently that IAPs are themselves controlled by ubiquitin-mediated degradation. For example, HIAP-1 is a direct target for HIAP-2-mediated ubiquitination and proteosomal degradation [Conze et al. 2005]. The RING domain of XIAP also mediates polyubiquitination of TAK-1, an enzyme responsible for activation of the pro-apoptotic kinase, JNK [Kaur et al. 2005]. In this fashion, XIAP is able to target TAK-1 for degradation by the proteosome thereby preventing JNK-mediated apoptosis. Structurally similar, both HIAP-1 and HIAP-2 possess a CARD, a highly conserved domain noted to promote homodimerization and oligomerization with other CARD containing proteins. While elevated expression of XIAP, HIAP-1, HIAP-2 and survivin in mitogen-stimulated T cells from patients with active RRMS correlates with clinical features of disease activity, deficits in Fas mediated cell death [Comi et al. 2000] and T cell resistance to apoptosis [Sharief et al. 2002b; Sharief and Semra 2001], a systematic examination of the expression patterns of these genes in whole blood, PBMN and resting T cells in patients with various forms of MS has yet to be done. Given that the failed apoptosis of auto-reactive T cells has been implicated in MS pathogenesis and that MS is a clinically heterogeneous disorder [Chofflon 2005], it would be advantageous if specific patterns of IAP expression in different immune cell subtypes could be measured and correlated with distinct forms of the disease. Furthermore, it would be highly advantageous to develop a reliable, rapid and inexpensive diagnostic test for multiple sclerosis subtypes based on specific patterns of basal IAP gene expression in peripheral immune cells. Finally, a diagnostic test would allow clinicians to decide whether interferon drug treatment is appropriate for a specific disease subtype.